I teach at a school and I create PowerPoint presentations to use in my classroom. I also need to upload the PowerPoints to my school website for my students to access. Access is restricted and students must sign in with their school codes. Can I use images I find on Google search? Are all images copyrighted? If they do not have an "ownership" stamp on them, are they ok to use in this manner? Nothing is being sold and the presentations are taken down at the end of each year.
Images found in Google search don't belong to Google.
I can't speak to fair-use under academia, but I can say images aren't fair game just because they're found on Google images
There are, however, places you can find images you can use. Wikimedia has a ton of images that can be used, with a wide variety of licenses for re-use. Flickr is another that comes to mind.
I want to reiterate that I don't know fair use under the realm of academia, but would ask permission and highly recommend captioning photos in order to give credit where it's due.
You ask a really good question. Many people download images from websites without even thinking about whether they can consume it. Now if you properly credit/cite your source, then you avoid being called a plagiarist at your school. However, you're not in the clear with the owner of that image. Basically, you need to find out what the author/artist requires. Some say you only need to credit and provide a link back. Others might not say anything, which in that you need to get permission. Crediting is not enough. My suggestion is when you do an image search, add the term "public domain" or similar. Or... I know this is not what you want hear, but actually purchase image bundles that clearly says unrestricted use. If you find images that have no clear author or owner, I would not use them. Those might be stolen. Or you can go to stock photo sites and buy images. That's what I do, by the way. You get unlimited and unrestricted (mostly) use. You'll sleep better at night knowing that the image police won't track you down someday.
You should not just use any image you come across online. In general you do not have permission. If in doubt, you do not have permission to use the image. Copyright is basically automatic, with more permissive choices needing to be actively labelled. However, you can limit a google image search to images with certain permission.
Another aspect to consider is 'fair use'. As an educator, roughly speaking you can make use of materials for the purpose of teaching a specific point. That is, use for learning purposes is probably acceptable (subject to details). Use of images that are purely for aesthetic purposes is not.